Recently, commercialization of an IC card system has been actively proceeded in various industries in Japan. For example, in the telephone industry, IC telephone cards have already been on sale and public telephones complying with IC cards have already been installed. Also, in the railway industry, an IC commutation ticket system is scheduled to be put in practical use in a few years. Furthermore, introduction of IC cards as substitution for identification cards such as a license and a health insurance card is under examination.
IC cards are classified into contact cards and contactless cards. In a contact IC card, an external terminal is brought into contact with a reader/writer so as to send/receive data and be supplied with power. In contrast, in a contactless IC card, data are sent to and received from a reader/writer and power is supplied from a reader/writer in a contactless manner by using a self-contained antenna. Furthermore, contactless IC cards under development in various companies are to comply with the anti-collision function of ISO 14443-3 for allowing one reader/writer to simultaneously write/read data in/from a plurality of IC cards. Owing to this anti-collision function, an effect that, for example, exchange of information on medical examination results and payment for the examination can be simultaneously completed by simultaneously subjecting a contactless IC card for hospital use and a contactless IC card for financial use to a reader/writer can be expected.
In a contactless IC card, since power is supplied through radio connection, the power supply is less stable than in a contact IC card. Therefore, in receiving a data from a reader/writer to store it in a self-contained nonvolatile memory or in reading a data from the nonvolatile memory to send it to a reader/writer, the data may not be normally received/sent due to the influence of noise caused by the operation of the nonvolatile memory. Furthermore, also in the case where one reader/writer simultaneously writes/reads data in/from a plurality of IC cards, data transmission of one IC card may not be normally conducted because of the influence of noise caused in another contactless IC card in storing a received data in the nonvolatile memory or sending a data read from a nonvolatile memory to the reader/writer.